• The last 80 years has seen considerable controversy about the classification of patients with mood-incongruent psychotic affective illness (MICPAI). Four viewpoints can be articulated: (1) MICPAI is indistinguishable from typical forms of affective illness; (2) MICPAI is a distinct subtype of affective illness; (3) MICPAI is a form of schizoaffective illness; and (4) MICPAI is a form of schizophrenia. Using the concept of diagnostic validators, I reviewed the empirical evidence for the validity of these four viewpoints. The available evidence argues relatively strongly against the first and fourth viewpoints and rather less strongly against the third. Data from diagnostic validators tend to support the second viewpoint, which is that taken by the framers of DSMIII and DSM-III-R.